This invention relates to the thermal treatment of any type of glass and glass-like materials, preferably of a glass sheet for shaping, bending, tempering, annealing, coating and float processing by rapidly and uniformly heating the glass sheet with microwave radiation so that the glass sheet can be processed without cracking. Glass and glass-like materials which may be thermally treated by the inventive method include flat glass sheets, glass fibers, organic mixtures incorporating glass and glass-like materials and the like. Glass sheets treated by this method can be used in the production of windshields, side windows and rear windows in vehicles such as automobiles and the like as well as for the production of architectural window glass and the like.
Thermal treatment of glass and glass-like materials is widely used for the production of vehicle windows, architectural glass, fiberglass ceilings and composites is and the like. A huge number of glass and glass-like materials and especially glass sheets are thermally treated each year worldwide.
One of the main problems in the thermal treatment of glass (e.g., the shaping or bending of glass sheets), is that the increasing heating rate of these glass sheets must be kept comparatively low when the glass is heated from room temperature to a softened temperature to prevent the glass sheet from cracking. Otherwise, cracking of the sheet can occur if different areas of the glass sheet are heated to different temperatures. This temperature differential between different areas or layers of the sheet raises the internal stresses in the glass to the point where these stresses become higher than the sheet modulus of rupture of the heated glass causing the glass to crack or shatter.
Generally, glass sheets are thermally treated by; conveying the sheets via an appropriate support mechanism through a horizontal tunnel-type furnace; heating them while in the furnace using infrared, hot air, gas or a combination of these methods to a heat softened temperature; and then shaping or bending the glass sheets. After shaping, the glass sheet is transferred to a cooling station where the sheet is controllably cooled. The described process assures that the thermal treatment is conducted at appropriate production rates. Numerous patents (see, e.g., U.S. Pat. Nos. 5,882,370, 5,858,047, 5,743,931, 5,352,263, 5,022,908, 5,079,931, 5,078,774, 5,066,320, 5,059,233, 5,057,138, 5,057,137, 5,032,162, 5,005,318, 4,986,842, 4,983,201, 4,976,762, 4,881,962, 4,816,055, 4,767,439 and 3,951,634) describe different methods of thermally treating glass sheets using tunnel-type furnaces. In all of these methods, the total heating time for each sheet while resident in the furnace is hundreds of seconds.
Productivity of tunnel-type furnaces can be increased, but only in limited ways. The simplest way to increase productivity is to make the furnace longer. A longer furnace allows the conveyer""s speed to be increased because the total heating time for each sheet is correspondingly increased, allowing the necessary low temperature differential between the external surfaces and inside layers of the glass sheet to be maintained. However, even without this modification, existing furnaces are too long, massive, expensive and inefficient and have material handling problems.
For example, there is high heat conduction from rollers to glass in tunnel-type furnaces, which together with the convection and radiation heat below the glass results in the amount of heat transferred from the lower surface of the glass exceeding that transferred to the upper surface of the glass through convection and radiation alone. In addition, there is non-uniformity of the glass temperature in the conveying direction that leads to deterioration of the surface quality and optical quality properties of the glass sheets.
Even if the glass sheet is evenly heated within this type of furnace, when the glass sheet is delivered out of the furnace, the leading and trailing ends of the glass sheet are cooled for different periods of time before the glass sheet reaches the pressing/bending position in the shaping/bending device. This can result in cracks in the glass sheet when it is pressed and bended.
Additionally, it is often difficult to reliably accomplish local heating for combined shaping and bending of glass sheets especially in auto and structural glass production when a massive furnace is used. These localized heating operations require expensive furnace modification (see, e.g., U.S. Pat. Nos. 5,735,922, 5,591,245 and 5,755,845) and still cannot guarantee the effective overheating of the limited locations and temperatures needed for bending.
Rather than eliminate the use of tunnel-type furnaces in the thermal processing of glass sheets, most improvements in the art have focused on reducing the size of the furnaces (which reduces the total heating time) without reducing production rate. One of the ways to achieve this goal is to increase the power of the heat applied to the glass at the moment the glass temperature exceeds around 450xc2x0 C. by creating a separate chamber at the end of the furnace heated by powerful electric or gas heaters having a temperature range of 800xc2x0 C. to 900xc2x0 C. (U.S. Pat. No. 5,232,482) or 800xc2x0 C. to 1000xc2x0 C. (U.S. Pat. No. 5,306,324). Microwave energy has also been used to reduce the total heating time by treatment with microwave radiation only at the end of the glass heating process starting at a temperature around 420-450xc2x0 C. (U.S. Pat. Nos. 4,838,915, 4,471,192, 5,656,053 and 5,822,879).
Another way to reduce furnace length and heating time is to increase heat uniformity. Many patents focus on the solution of problems related to the non-uniform heating of glass including designing heating elements that are separately and independently controllable and which create the necessary distribution of heat by joining infrared and hot air heating (U.S. Pat. Nos. 5,908,000, 5,368,624 and 5,296,270), designing special heating devices (U.S. Pat. Nos. 6,005,230 and 4,888,038), creating a special temperature gradient over a glass surface (U.S. Pat. No. 5,149,352) and using furnace roller heat (U.S. Pat. No. 4,591,374).
However, as noted above, the common feature of all of the abovementioned patents is that they describe methods to reduce the total heating time of a glass sheet in tunnel furnaces and are not directed to (or capable of) eliminating this type furnace in glass processing. Thus, there is a clear need in the art for a method for the rapid heating of glass sheets which eliminates tunnel type furnaces or which shortens their length considerably to make them more effective and less expensive. A need also exists for a treatment method that can ensure high-speed heating of narrow, localized places on a glass sheet.
The main problem with increasing the rate of heating is the inevitability of creating temperature differences between the interior and the external surfaces of the glass sheets. As discussed above, different patents describe the equalization of infrared or convection heat on glass surfaces. Microwave heaters can be employed for this purpose as well (see, e.g., U.S. Pat. No. 5,828,042, U.S. patent application Ser. No. 09/439,533 filed Nov. 12, 1999). Hypothetically, the methods described therein at least provide a way to keep this temperature differential to a minimum on the external glass surface or surfaces. However it is extremely difficult to reduce the temperature differential throughout the thickness of the glass sheet without changing its properties.
The unsuitability of conventional radiation sources for the rapid heating of a glass sheet is illustrated by the following example and FIG. 1. Glass highly absorbs infrared radiation and is opaque to hot air and gas as well. Thus, when infrared or convection heat 1 transmitted by hot air, a gas, or the like interacts with a glass sheet 2 the main portion of the power is absorbed by the thin layers of the glass located at the external surfaces of the glass sheet, 2a and 2z. This natural phenomenon creates a temperature differential (represented as temperature profile curves 3, 4 and 5) in the interior of glass sheet 2 between the external glass surfaces 2a and 2z and a plurality of interior glass layers (e.g., 2b, 2c, 2d, . . . 2n), as well as between different layers within the glass, such that the temperature of the most interior layers (e.g., 2n) is lower than the temperature at the external surfaces of the glass sheet, 2a and 2z. If the heating time is long enough (i.e. the rate of heating is low enough), the thermal conductivity of the glass sheet can lessen the temperature differential as depicted by temperature profile curve 3. But if the increasing heating rate is high, the temperature differential between the interior layers (e.g., 2c, 2n) and the external surfaces of the glass sheet, 2a and 2z, increases (temperature profile curve 4) and the higher rate of heating creates an even higher resultant temperature differential (temperature profile curve 5) between these layers. This temperature differential, in turn, give rise to an internal stress in the glass sheet that leads to glass breakage. The value of the temperature differentials mainly depends on the absorption properties of the glass for infrared or convectional radiation and its thermal conductivity and consequently limits the ability to heat with a high increasing heating rate.
Since the usual function of glass is to protect an object from infrared and heat To radiation, most kinds of glass and glass-like materials, especially window type glass, absorb infrared very well and have very low thermal conductivity. Thus, the time necessary for heating glass by heat sources that do not have the ability to penetrate inside the glass has a natural limit that cannot be reduced because this leads to breakage. This limitation in heating time can be estimated through the solution of a heat equation that describes the spread of heat inside a glass sheet that is irradiated, for example, by infrared radiation. This equation can be written in the following form (see for example Conduction of Heat in Solids by J. C. Jaeger (contributor), Horatio S. Carslaw, 2nd edition, Oxford University Pres, 1986):
∂T/∂t=xcex7/(cxcfx81)xcex94T+Q/(cxcfx81)
The heat transfer (q) from the external surfaces of the glass sheet can be written in the following form:
q=k(T(0,t)xe2x88x92T0)+xcex2"sgr"(T(0,t)4xe2x88x92T04),
where T=T(x,t) is the temperature distribution along the (x) axis (throughout glass thickness) for irradiation time (t) and 0 less than x less than xcex4, xcex4 is the thickness of the glass; T(0,t) is the temperature of the glass surface vs. time (t) starting from room temperature T0, xcex7 is the glass thermal conductivity, c is the specific heat of the glass, xcfx81 is the glass density, k is the coefficient of heat transfer, xcex2 is the total emissivity of the glass, "sgr"xe2x80x94Stefan-Boltzmann constant (see Handbook of Chemistry and Physics 80th edition, CRC Press LLC 1999), q is the flow of heat from the glass surface and Q=Q(x) is the distribution of absorbed energy along the (x) axis (along thickness) created in the glass while irradiated. The distribution of the absorbed energy across the glass sheet surfaces is assumed to be uniform.
From this heat equation, it is possible to estimate the minimal possible heating time (t), which creates the maximal temperature difference (xcex94Tmax) inside a glass sheet (i.e., the maximum temperature differential between the most interior glass layer and the external surface of the sheet) and the correspondent maximal internal stress ("sgr"Tmax) that would be low enough to avoid glass cracking or in other words "sgr"T max less than MoR, where MoR is the mean modulus of rupture and "sgr"Tmax=Ksxcex94Tmax (where Ks is the coefficient of thermal stress) (see, e.g., E. B. Shand, Glass Engineering Handbook. 2nd Edition, McGraw Book Company, Inc., New York, Toronto, London, 1958, pp. 112-117]
This time (t) can be estimated by calculating the temperature distribution inside the glass, multiplying the difference between the average temperature and the minimal (or maximal) temperature (xcex94Tmax) by the thermal stress coefficient (Ks) and comparing this value to MoR. These calculations can be made for different power densities and requencies (in fact, not only for the infrared frequency range) of radiation applied to glass.
The calculations are based on the following properties of soda-lime float glass at room temperature with a thickness (xcex4) of 4 mm and are taken from E. B. Shand, Glass Engineering Handbook 2nd Edition, McGraw-Hill Book Company, Inc., 1958, pp. 23, 25, 28 and 112-113 and Physical Properties of Glass, J. E. Stanworth, Oxford, Clarendon Press, 1953, pg. 105: Soda-lime float glass is the glass used in this example because it is the type of glass that is most widely used in the production of automotive and architectural glasses.
xcex7=0.937W/(m xc2x0 C.);
c=0.21 cal/(g xc2x0 C.);
xcfx81=2530 kg/m3;
Ks=0.62 MPa/xc2x0 C.;
MoR=41 MPa.
In the exemplary calculations, Tmax is selected at 640xc2x0 C., room temperature To is selected as 27xc2x0 C. The calculations are made for a glass sheet heated by infrared radiation with a mean wavelength of  greater than 4 microns. Infrared radiation with a power density I0 heats the glass from both sides and penetrates into the glass only to the depth, d less than 0.3 mm (see for example, E. B. Shand, Glass Engineering Handbook, Second Edition, McGraw Book Company, Inc., New York, Toronto, London, 1958, p 62). Q(x) (i.e., the distribution of absorbed energy along the (x) axis in the irradiated glass) is calculated as:
Q(x)=(I0/d){exp(xe2x88x92x/d)+exp(xe2x88x92(xcex4xe2x88x92x)/d)}.
The results of these calculations are shown in an alignment chart below (see Table 1).
The data presented in Table 1 illustrate that even when the external glass surfaces are heated absolutely uniformly by infrared radiation, there is a lower limit to the heating time (i.e. dozens of seconds) and the corresponding increasing heating rate is lower than around 12xc2x0 C./s (for heating glass from room temperature). This result is applicable to convection heat as well, or to its combination with infrared radiation. In fact, the actual increasing heating rate is significantly lower. For example, for a 4 mm glass sheet, the rate of heating is not higher than 6xc2x0 C./s-8xc2x0 C./s because of the problems experienced in uniformly heating the glass surfaces. But, even in the best case, the limitation in the heating time (the increasing heating rate) makes the use of long tunnel type furnaces inevitable, as confirmed by worldwide industrial practice.
Thus, the time required for heating glass by common infrared radiation or any convection heat source is too long for the elimination of tunnel type furnaces and it is impossible to reduce this heating time without changing the glass properties. Changing these properties requires increasing the glass transparency to infrared radiation and increasing the glass thermal conductivity. However, this solution is impractical because glass windows that are highly transparent to infrared or convectional radiation have high thermal conductivity and do not protect from outside heat and cold. Consequently, such glass windows are virtually worthless.
According to the present invention, a method is provided for the thermal treatment of any glass or glass-like material that is processible by controlled heating without using conventional tunnel-type furnaces. Glass-like materials include those materials that contain glass powders, glass threads and the like. These products prepared using these treatments include but are not limited to windshields, side windows and rear windows for vehicles such as automobiles and the like as well as architectural glass, products made from fiberglass and comparable products. The inventive method utilizes microwave radiation to rapidly heat the material to be thermally treated (e.g., a glass sheet) starting from any initial temperature. It is understood that the initial temperature of the glass sheet includes any cooled temperature below room temperature, room temperature and any temperature above room temperature up to the softening temperature of the particular glass treated.
The inventive method avoids the use of bulky and inefficient tunnel-type furnaces and is particularly applicable to glass shaping, bending, tempering, annealing, coating, floating, and the like. Architectural window glass and similar materials can also be thermally heated using this method, as well as other glass items where thermal treatment of glass sheets is required. The inventive method can also be used to volumetrically dry fiberglass and other inorganic materials with low thermal conductivity and which are non-transparent to infrared and convectional heat. Once the glass sheet is thermally treated using the inventive method, other known processing steps can be employed to complete the shaping, bending, tempering, annealing, coating or floating of the glass sheet. These steps are exemplified in the U.S. Patents set forth in the BACKGROUND OF THE INVENTION and are hereby incorporated by reference.
U.S. Pat. No. 5,827,345 recognized that if a glass sheet is below its softening point, the application of microwave energy may break the glass sheet. Indeed, in actual practice, microwave radiation has not been used to heat glass sheets from room temperature to a temperature suitable for the production of large size glass-based products such as windows, mats and the like.
The inventive method differs from previous methods for the thermal treatment of glass using microwave radiation in that the inventive method enables the whole glass sheet or any localized areas therein to be safely (i.e., without cracking) and controllably heated from any initial temperature to any required temperature (including a softened temperature) in a short selected time with high efficiency. Other differences between the previously used microwave-based heating methodologies and the inventive method are set forth in the DETAILED DESCRIPTION OF THE INVENTION and include inter alia these features of the inventive method; the selection of optimal wavelengths (frequencies, including those corresponding to the thickness of the irradiated glass sheet) for the applied microwave radiation, the equalization of the power (temperature) distributions in the interior of the glass sheet during microwave heating, the continuous adjustment of the applied microwave power density with the increasing temperature of the glass sheet (where increasing temperature is correlated with changing glass properties), the definition of the optimal microwave power distribution uniformity (which is required for high speed heating) and the heating of the glass sheet through the shaping/bending tool as well as the heating of the tool itself.
The main advantages of this high-speed method are reducing manufacturing costs and increasing production rate. Many other specific advantages also exist s including but not limited to the elimination of the cost and issues of transporting and transferring hot glass to, for example, a press site, compacting the size of the apparatus, increasing quality of the final product after annealing, shaping, bending, annealing and tempering and simplifying the shaping/bending tool.